In the health field, medical equipment such as kidney dialysis machines, infusion pump blood analyzers, transfusion systems, cardio-pulmonary bypass/assist machines, and the like, include, or are attached to, tubes in which a patient's blood flows, or in which some form of infusion fluid flows. These fluids flow from these tubes into the patient's blood stream by means of a cannula, and it is important for patient care, and to ensure patient safety, that the flow rate of the fluid is appropriately monitored. It is also important to measure the temperature of these fluids entering the patient's body so as not to put undue stress on the patient with infusion of fluids that are substantially warmer or colder than target body temperatures, whether physiologic or therapeutic. In some cases, the infusion of fluids may have a fluid temperature that is substantially above, or substantially below, physiologic body temperature (e.g., therapeutic cooling during heart surgery to 18° C. followed by rewarming at the end of surgery). In such instances, the need to monitor temperature of the fluids in these cases is greater.
It is also important to monitor fluid flowing in these tubes for undesirably large gas bubbles, whether air or some other gas, because such gas bubbles have the potential to harm the patient as emboli when they enter the patient's bloodstream. One solution with respect to the monitoring of fluid flow rate, fluid temperature, and the monitoring for gas bubbles in the flowing fluid, is to provide separate sensors and associated electronic circuits for each of these parameters, namely, fluid flow rate, fluid temperature, and the presence of substantial gas bubbles. However, a disadvantage to such systems would be their complexity of construction, which makes them more costly to manufacture, deploy and maintain.
A partial solution to this problem is disclosed by U.S. Pat. No. 7,661,294 B2, issued to Dam, and which is incorporated herein by reference in its entirety. According to this Dam patent, a multi-function sensor system may be constructed with piezoelectric elements that are operated as part of an air bubble detection and characterization apparatus. Dam further discloses that the multi-function sensor system includes an infra-red thermocouple employed as a temperature sensor element operated to measure the internal temperature of liquid in a tube non-invasively by measuring both the tube surface temperature and ambient temperature. The multi-function sensor system disclosed by Dam includes a force/pressure sensor that accomplishes non-invasive measurement of internal pressure of an elastic tube to detect tube occlusion and/or disconnections.
The Dam patent discloses a liquid color sensing circuit that employs a light emitting element and phototransistor to sense the color of liquid in a tube. However, Dam does not disclose a circuit for detecting hematocrit and/or hemoglobin of blood in a tube. It is known that hematocrit and/or hemoglobin of blood may affect blood flow measurements, so there is a need for a blood flow sensing apparatus that senses blood flow rate more accurately in a tube or pipe by correcting for the effect of hematocrit/hemoglobin and/or the effect of temperature on the blood flow measurements.
There remains a need for a compact, easy to deploy and use apparatus that senses fluid flow rate and that detects the presence of gas bubbles for fluid flowing in a tube or pipe. Furthermore, there remains a need for an apparatus that senses fluid flow rate more accurately than previous fluid flow sensing apparatuses.